This invention relates to a light alloy piston for internal combustion engines, which piston has a convex, oval external shape and comprises a plurality of load-carrying skirt surfaces each of which is divide each into a plurality of load-carrying skirt surface portions.
Such a light alloy piston has been disclosed in the Published German patent application No. 34 37 117. In that known light alloy piston each of the load-carrying skirt surfaces is divided by a depression, which extends in the peripheral direction of the piston, into two skirt surface portions, namely an upper skirt surface portion and a lower one. The depression extends between the clearance-defining upper and lower portions of the sliding surface of the skirt and has a depth which is about twice the peak-to-valley height of the scored outside surface of the skirt. That depression has such an axial height that it will not merge into the cylinder when the piston is at its bottom dead center and that in the cold engine the outside surface of the skirt has between the upper limit of the depression and the lower limit of the clearance-defining upper portion of the sliding surface of the skirt a load-carrying surface portion, which is parallel to the axis of the piston and has an axial height of 1.5 to 4.5 mm, preferably 1.5 to 2.5 mm, and the width of the load-carrying skirt surface portions and the width of the depression are at least as large as the width of the area in which a pressure mark will be formed on the outside surface of the skirt.
That design of the skirt of the piston will improve the floating on the lubricant film so that the losses due to friction will distinctly be reduced and improved sliding properties will be obtained. Having a relatively small depth, the depression constitutes an excellent lubricant reservoir, which contributes to the maintenance of the hydrodynamic lubrication of the load-carrying portions of the skirt. A special advantage resides in that the pressure per unit of area will not increase with the load because the skirt surface portions which will contact the cylinder during an operation under a partial load are relatively narrow and the width of said surface portions progressively increases with the load until they extend on a given half of the piston skirt in a peripheral direction over an arc of as much as 90.degree. . The necessary guidance of the piston in the cylinder along a straight line will be ensured under all operating conditions.
It has now been found that a guidance of the light alloy piston with the required accuracy will not always be ensured when the scored surface of the cold piston defines a large radial clearance adjacent to the top end of the piston skirt owing to the design of the piston and/or owing to the high temperatures in that region and in order to produce good lubricating conditions. In that case the piston will be guided substantially only by the lower surface portion of the piston skirt as the engine is warming up.